Cellulosic pulp based products comprise one of the largest and most important markets for commercial materials. The technology involved with paper and cardboard is well developed and comprises many additives to yield a multitude of property improvements. Property improvements desired include wet and dry tensile strength, wet and dry burst strength, wet and dry tear resistance, fold resistance, oil resistance, solvent/stain resistance, etc. Additives to paper are characterized by the position of addition relative to the paper-making process. The addition of additives to the slurried pulp (paper stock) prior to sheet formation is commonly referred to as wet-end addition. The addition to paper after formation and at least partial drying is referred to as dry-end addition.
Various additives are applied to the pulp slurry prior to sheet formation. These include retention aids to retain fines and fillers (e.g. alum, poly(ethyleneimine), cationic starches), drainage aids (e.g. poly(ethyleneimine), defoamers, additives which control pitch or stickies (e.g. microfibers, adsorbent fillers). Additionally wet strength additives such as cationic polyacrylamides and poly(amide amine/epichlorohydrin) are added in the wet end to improve wet strength as well as dry strength. Starch, guar gums, and polyacrylamides are also added to yield dry strength improvements. Urea-formaldehyde and melamine-formaldehyde resins are employed as low cost wet strength additives; however, due to residual formaldehyde these resins have fallen out of favor and are being replaced.
Sizing agents are added to impart hydrophobic character to the hydrophilic cellulosic fibers. These agents are used for liquid containers (e.g. milk, juice), paper cups, and surfaces printed by aqueous inks (to prevent spreading of the ink). Rosin sizes derived from pine trees were initially used as well as wax emulsions. More recently, cellulose-reactive sizes have been employed. These include alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA). AKD is discussed b Marton (TAPPI J., p. 139, Nov. 1990) and Zhou (Paper Technology, p. 19, Jul. 1991).
The additives noted above can also be added to the dry-end of the papermaking process. These additives can be added various ways. One of the common methods is referred to as the size press addition. This generally involves nip rolls in which a water solution of the additive(s) is flooded and allowed to penetrate the paper. Other methods of addition include spray application and tub sizing.
Starch is the most commonly employed additive in size press addition. Carboxy methyl cellulose, polyvinyl alcohol, cellulose reactive sizes, wax emulsions are also commonly employed for size press addition. Poly(vinyl acetate) emulsions, as well as poly(ethylene-vinyl chloride), poly(styrenebutadiene) and polyacrylic emulsions are commonly added at the dry-end of the paper making process as a surface size or paper coating additive. The add-on levels (dry additive on dry pulp) at the dry end can be low (0.05-4 wt%) as sizing additives (either surface or internal sizing) or high (4-20+wt% dry-on-dry) in the case of saturation sizing. The properties desired are variable, however, include wet and dry tensile strength, fold resistance, wet and dry burst strength, porosity closing, wet and dry tear strength, printability, surface characteristics, oil resistance, etc.
Specific versions of poly(vinyl alcohol) offer many of these improvements, specifically dry strength, wet strength, fold resistance, burst strength and oil resistance. Poly(vinyl alcohol) is generally added in dry-end application as it has poor substansivity to cellulosic products. Highly crystalline poly(vinyl alcohol) generally yields the best wet strength properties as it is insoluble in cold water. Crosslinking additives such as glyoxal can be added to yield specific property improvements. (See Polyvinyl Alcohol Developments, C.A. Finch, ed. (1992) pp 270-273; 591-595).
The use of functional polymers of various types has been known for many years as a means to improve papermaking processes and paper properties. Several of these resins for improving wet strength of the paper have involved products derived from epihalohydrin. U.S. 3,535,288 Lipowski, et al. (1970) discloses an improved cationic polyamide-epichlorohydrin thermosetting resin as useful in the manufacture of wet-strength paper. U.S. 3,715,336 Nowak, et al. (1973) describes vinyl alcohol/vinylamine copolymers as useful flocculants in clarification of aqueous suspensions and, when combined with epichlorohydrin, as useful wet-strength resins for paper. The copolymers are prepared by hydrolysis of vinylcarbamate/vinyl acetate copolymers made by copolymerization of vinyl acetate and vinyl isocyanate followed by the conversion of the isocyanate functionality to carbamate functionality with an alkanol. Additionally, Canadian Pat. No. 1,155,597 (1983) discloses wet-strength resins used in papermaking, including polymers of diallylamine reacted with epihalohydrin and a vinyl polymer reacted with epihalohydrin wherein the vinyl polymer is formed from a monomer prepared by reacting an aromatic vinyl alkyl halide with an amine, such as dimethylamine.
Functional polymers derived from amides have also been used to improve paper processes. U.S. 3,597,314 Lanbe, et al. (1971) discloses that drainage of cellulose fiber suspensions can be enhanced by the addition of a fully or partially hydrolyzed polymer of N-vinyl-N-methyl carboxylic acid amide. U.S. 4,311,805 Moritani, et al. (1982) discloses paper-strength additives made by copolymerizing a vinyl ester, such as vinyl acetate, and an acrylamide derivative, followed by hydrolysis of the ester groups to hydroxy groups. The presence of the remaining cationic groups enables the polymer to be adsorbed on pulp fibers. Utilities for the polymers as sizing agents, drainage aids, size retention aids and as binders for pigments are disclosed but not demonstrated. U.S. 4,421,602 Brunnmueller, et al. (1983) describes partially hydrolyzed homopolymers of N-vinylformamide as useful as retention agents, drainage aids and flocculants in papermaking. European patent application 0,331,047 (1989) notes the utility of high molecular weight poly(vinylamine) as a wet-end additive in papermaking for improved dry strength and as a filler retention aid. U.S. Pat. No. 4,614,762 discusses a water soluble product of polyethyleneimine reacted with formaldehyde and poly(vinyl alcohol). The product is noted to be useful as an improved drainage and retention aid in papermaking.
More recently, vinylamide copolymers have been disclosed as useful in papermaking to improve the properties of the product. U.S. Pat. No. 4,774,285 Pfohl, et al. (1988) describes amine functional polymers formed by copolymerizing vinyl acetate or vinyl propionate with N-vinylformamide (NVF) followed by 30-100% hydrolysis to eliminate formyl groups and the acetyl or propionyl groups. The copolymer contains 10-95 mole, NVF and 5-90 mole% vinyl acetate or vinyl propionate. The hydrolyzed copolymers are useful in papermaking to increase dry strength and wet strength when added in an amount of 0.1 to 5 wt% based on dry fiber. The polymer can be added to the pulp or applied to the formed sheet. The two polymers used to show dry and wet strength improvements are said to contain 40% and 60% N-vinylformamide before hydrolysis. Lower levels of amine functionality in poly(vinyl alcohol) are not demonstrated to be effective.
U.S. Pat. Nos. 4,880,497 and 4,978,427 discuss the use of amine functional polymers for use in improving the dry and wet strength of paper. These amine functional polymers are based on copolymers comprising 10 to 95 mole % N-vinyl formamide which are hydrolyzed to yield amine functionality. The copolymer also contains an ethylenically unsaturated monomer including vinyl esters (such as vinyl acetate), alkyl vinyl ethers, N-vinyl pyrrolidone, and the esters, nitrites and amides of acrylic acid or methacrylic acid. The problems of copolymerization to yield uniform copolymers of vinyl acetate/N-vinyl formamide above 10 mole % NVF are not noted and the examples shown in these patents do not represent random copolymers but most probably polymer mixtures of various compositions between poly(vinyl acetate) and poly(N-vinyl formamide) (before hydrolysis).
U.S. Pat. No. 4,808,683 Itagaki, et al. (1989) describes a vinylamine copolymer such as a copolymer of N-vinylformamide and N-substituted-acrylamide, which is said to be useful as a paper strengthening agent and European patent application 0,251,182 (1988) describes a vinylamine copolymer formed by hydrolysis of a copolymer of N-vinylformamide and acrylonitrile or methacrylonitrile. The product is said to be useful in papermaking as a drainage aid, retention aid and strength increasing agent. Examples presented to demonstrate the paper strengthening effect of the polymer used a pulp slurry containing cationic starch, alkyl ketene dimer as a sizing agent and a filler retention improving agent, but there is no indication of any cooperative effect between the polymer and the sizing agent.
On the other hand, certain combinations of additives have been found to be useful as paper additives. U.S. Pat. No. 4,772,359, Linhart, et al. (1988) discloses utility of homopolymers or copolymers of N-vinylamides, such as N-vinylformamide (NVF), in combination with phenol resin as a drainage aid in pulp slurries for production of paper. In this service unhydrolyzed poly NVF is said to function cooperatively with the phenol resin, while a partially hydrolyzed poly NVF does not (see Example 6). European patent application No. 0,337,310 (1989) describes improving moist compressive strength of paper products using the combination of hydrolyzed poly(vinyl-acetate-vinylamide) and an anionic polymer such as carboxymethyl cellulose or anionic starch. The hydrolyzed polymer can contain 1-50 mole% vinylamine units and examples are given of polymers having amine functionality of 3-30%.
G. G. Spence in Encyclopedia of Polymer Science and Technology, 2nd Ed., Wiley-Interscience, Vol. 10, p. 761-786, N.Y., 1987, provides a comprehensive survey of paper additives describing the functions and benefits of various additives and resins used in the manufacture of paper. Wet-end additives are discussed at length. Resins containing amine groups that provide cationic functionality and have low molecular weights (10.sup.3 to 10.sup.5) e.g., poly(ethyleneimine), are used to aid retention of fines in the paper. Acrylamide-based water soluble polymers are used as additives to enhance dry strength of paper while a variety of resins, such as melamineformaldehyde resins, improve wet strength. Poly(ethyleneimine), however, is said not to be commercially significant as a wet-strength resin. Sizing agents are used to reduce penetration of liquids, especially water, into paper which, being cellulosic, is very hydrophilic. Sizing agents disclosed are rosin-based agents, synthetic cellulose-reactive materials such as alkyl ketene dimer (AKD), alkenyl succinic anhydrides (ASA) and anhydrides of long-chain fatty acids, such as stearic anhydride, wax emulsions and fluorochemical sizes. Cationic retention aids, such as alum, cationic starch or aminopolyamide-epichlorohydrin wet-strength resin, are used to retain the size particles in the sheet.